Return of G K 400 RB.

[catstrat]

HI Jazz, i'll post them if i can figure out how to do it. I just checked Q10 which is a driver and i have - 59 volts on the B and the E. I checked that trans out of circuit yesterday and it tested good. I'm pulling it to check it again. I checked voltage on both ends of the bias trim pot and i have -59 volts all along the bias circuit.

[catstrat]

400 rb

Let me see if this worked on poasting the files

[catstrat]

The layout is for the 1992 year and the schematic i have is the proper schematic for 1985. The layout is different but i'm not sure how different. You can compare the layout here to the files OC Disorder posted. I did pull Q 10 and Q4 for grins and they both test good out of circuit.

[52 Bill]

Alright. Q2 is a 15031 and it's mounted the the heatsink. The collector leg is clipped since the collector connects to the heatsink.

No, the collector leg is clipped because the tab is connected to the circuit by the mounting screw. It must be insulated from the heatsink with a mica insulator.

All of the transistors that are mounted on the heatsink should be insulated from touching it.

[g1]

52Bill has it, the heatsinks are grounded and none of the collectors should be.

[catstrat]

400 rb

Thanks Bill. i stand corrected. Each transistor has an insulator. I did my best to look closely at them when i replaced transistors but they tell me even a small pinhole can cause problems. I've got some insulators in stock so maybe it's a good idea to go ahead and change them all. That may be the problem. Should i use a new insulator each time i change a transistor? I' ve heard different opinions on this as well. Thanks. I hope this info solves it because i didn't even think about the insulators.

[Dave H]

If I replace any output transistors I always check with a meter to make sure none of the collectors is grounded before switching on.

What is the output voltage of the opamp when the output is -57V?

And why does the schematic say the output is 41V no load?

[catstrat]

HI Dave. I wondered about the 41 volt flag on the schematic to. At the bottom right of one of the schematics it has a rectangle drawn with a reference to RMS. I was thnking that 41 will be A C voltage during test, or is it? I haven't tested this under load with the - 57 volts at the O/p.Would the amp work right with -57 volts at the o/p? I don't think so but i'm not expert on solid state either.

[Enzo]

The 41v at the output is AC volts of signal there. There should be no DC on the output.

[52 Bill]

HI Dave. I wondered about the 41 volt flag on the schematic to. At the bottom right of one of the schematics it has a rectangle drawn with a reference to RMS. I was thnking that 41 will be A C voltage during test, or is it? I haven't tested this under load with the - 57 volts at the O/p.Would the amp work right with -57 volts at the o/p? I don't think so but i'm not expert on solid state either.

The 41 volts ac is the signal voltage at the output when driven without a load.

When the amp is working properly there should be no dc voltage on the output other than perhaps a few millivolts of offset. In really rough terms, the top half of the circuit amplifies the top half of the signal and the bottom part of the circuit amplifies the bottom half of the signal. Just like the concept of a push pull tube output stage.

If there is a dc voltage on the output, then either some part is shorted or not working properly to get the top and bottom parts of the circuit in balance. When both parts of the circuit are balanced, there will be zero dc volts at the midpoint.

So, do not connect a speaker to the output for testing while you still have any dc voltage on the output terminals. It will only cause additional damage to components in the amp.

[catstrat]

400 rb

I had to set the 400 rb to the side for now because i have a Fender and a Marshall the customer wants ready by Wednesday. I think Bill you may have steered me in the right direction on the insulators. I did look at them closely but i just didn't think about them being the problem. I feel kind of dumb actually. I should be able to remove each transistor one at a time and try to find the one shorting. I admit, they do looked pretty old.

[g1]

Along with the insulators, the plastic shoulder type bushings on the transistor screws are critical.

[Dave H]

The 41v at the output is AC volts of signal there. There should be no DC on the output.

Thanks Enzo. So all the voltages in the square boxes are AC? I searched the schematic for a note saying what they were but didn't find one.


Catstrat,

Check the output transistors for collector shorts to ground. If there are none then with no output load check to see if you can still adjust the bias current. Measure the voltage across one of the emitter resistors (R24,25,26,27) and set it to 5mV (15mA bias current) then check that the voltage across the other three emitter resistors is also 5mV. If that's OK then your output transistors are fine. Check the +/-15V supplies to the opamp U1 (pin 7 + pin 4 -) then measure its output (pin 6). It should be hard -ve (-13.5V) which will turn Q1 and Q2 hard on and with the amp's output being at -57V Q2 should be stinking hot. If it is then Q5 could be short circuit (probably killed when Q7 shorted). If Q2 is cold then Q2 or R11 could be open circuit. If the opamp's output is not hard -ve it's probably dead.

[g1]

So all the voltages in the square boxes are AC? I searched the schematic for a note saying what they were but didn't find one.

The AC signal voltages are boxed, the DC are not.
The note is probably on the preamp schem., along with input signal voltage and control settings?

[Enzo]

Good practice would be to have a legend on the drawing to explain all symbology, but in its absence, those being AC should be understood. No amp would have 41vDC on its output, even a single sides amp like an old Acoustic would have a large cap in the speaker line. And looking over at the input IC I see -7v at the output pin as well as 2.5v there. The polarity makes the 7v as DC, and 2.5v there with 0.65v at the IC input makes them sure bets as AC.



Solid state amps are just amps, after all. You plug a speaker into one. Just as with a tube amp. You certainly would not think +41v was OK on a tube amp. so why would it be OK on a solid state amp?